Carbon-modified TiO2 for photocatalysis
© Wojtoniszak et al; licensee Springer. 2012
Received: 25 January 2012
Accepted: 26 April 2012
Published: 26 April 2012
Here we present a method to produce TiO2 nanocrystals coated by thin layer of graphitic carbon. The coating process was prepared via chemical vapor deposition (CVD) with acetylene used as a carbon feedstock with TiO2 used as a substrate. Different temperatures (400°C and 500°C) and times (10, 20, and 60 s) of reaction were explored. The prepared nanocomposites were investigated by means of transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy/diffuse reflectance spectroscopy and ultraviolet-vis (UV-vis)/diffuse reflectance spectroscopy. Furthermore, photocatalytic activity of the materials was investigated under visible and UV-vis light irradiation in the process of phenol decomposition. It was found that TiO2 modification with carbon resulted in a significant increase of photoactivity under visible irradiation and decrease under UV-vis light irradiation. Interestingly, a shorter CVD time and higher process temperature resulted in the preparation of the samples exhibiting higher activity in the photocatalytic process under visible light irradiation.
KeywordsGraphite CVD TiO2 Photocatalyst
Titanium dioxide generates a great interest in materials science due to its amazing photocatalytic performance, low cost, long-term stability, and promising application in photocatalysis areas such as wastewater  and air purification , degradation of brevetoxins in aqueous and organic media , or destruction of microorganisms in water . Unfortunately, due to its relatively high band-gap energy (3.0 eV for rutile and 3.2 eV for anatase), TiO2 can be excited only by ultaviolet (UV) light. That is why the modification of TiO2 towards shifting the absorption threshold to the visible light region in order to allow utilization of solar energy attracted attention of many researchers. Therefore, several methods of TiO2 modification have been proposed such as semiconductors coupling , doping , or fluorination . These methods resulted in enhanced photocatalytic activity under visible light irradiation.
Recently, graphene, a single-atom planar sheet of sp2-bonded carbon atoms, has attracted a great interest in the field of photocatalysis. The attention comes from its outstanding properties. For instance, graphene has a charge mobility which is among the highest of any other semiconductors [8–10] is the strongest material ever measured  and has a thermal conductivity more than double that of diamond . Furthermore, its high surface area to volume ratio makes graphene an ideal support for TiO2. It was found that titanium dioxide functionalized with graphene exhibits enhanced photocatalytic performance under visible and UV light irradiation in comparison to the pristine material . Graphene can improve an efficiency of photo-conversion since it may act as an electron transfer channel and inhibits a recombination of the electron-hole pairs. So far, many methods have been developed in order to synthesize graphene-modified TiO2 photocatalysts [14, 15]. Wang et al.  synthesized TiO2-graphene nanocomposite from melamine which was used as a precursor of graphene. In the report of Zhang et al. , graphene was obtained from graphene oxide and further used as TiO2 support in the nanocomposite. In our study, we developed a method to produce TiO2 /graphitic carbon nanocomposite using chemical vapor deposition (CVD). Here, CVD process of acetylene used as graphene source with TiO2 as a template was performed. The obtained material was further used as a photocatalyst in the phenol decomposition under UV-vis and visible light irradiation.
Pristine TiO2 was obtained from Police SA (Zaklady Chemiczne Police SA, Kuźnicka 1, Police, Poland). Acetylene 2.6 and argon 4.8 were purchased from Messer (Messer Polska sp. z o.o., Maciejkowicka 30, Chorzów, Poland). Phenol (99.5%, Sigma-Aldrich, Saint Louis, MO, USA) was used as a model organic pollutant. High purity water for the photocatalytic experiments and sample analysis was produced by Millipore Elix advantage water purification system (Millipore Corporation, Billerica, MA, USA).
CVD growth of graphene
The CVD processes were performed in a horizontal furnace with a quartz tube reactor. Pristine TiO2 powder was placed in an alumina crucible. Prior to the synthesis, the reactor was evacuated down to 1 hPa, after which a temperature was increased to 400°C or 500°C in argon atmosphere (600 sscm). Then, acetylene was introduced also with 600 sscm. Six experiments at 400°C and 500°C for 10, 20, and 60 s have been performed, respectively. After each reaction, the furnace was cooled down to room temperature in argon atmosphere.
The morphology of the obtained material was characterized via transmission electron microscopy (FEI Tecnai F30, Frequency Electronics Inc., Mitchel Field, NY, USA). Raman spectra were acquired on the inVia Raman Microscope (Renishaw PLC, New Mills Wotton-under-Edge, Gloucestershire, UK) at an excitation wavelength of 785 nm. The surface properties of the photocatalysts were examined by means of Fourier transform infrared spectroscopy/diffuse reflectance spectroscopy (FTIR/DRS). Measurements were performed using a Jasco FTIR 4200 (Jasco International Co. Ltd., Hachioji, Tokyo, Japan) spectrometer equipped with a diffuse reflectance accessory (Harrick, Bridgewater, NJ, USA). Thermogravimetric analysis (TGA) was performed on the SDT-Q600 TGA (TA Instruments Inc., Milford, MA, USA) under an air flow of 100 mL/min and at a heating rate of 5°C/min in order to estimate a quantitative composition of the photocatalysts. The photocatalysts were characterized by UV-vis/DR technique using a Jasco V-650 spectrophotometer (Jasco International Co. Ltd., Hachioji, Tokyo, Japan) equipped with an integrating sphere accessory for diffuse reflectance spectra acquisition.
Photocatalytic activities of the samples under UV-vis (> 290 nm) irradiation were evaluated by oxidative decomposition of phenol under air in aqueous solutions. The reaction was performed as described in previous publication , while the photocatalytic activity under visible light was tested during 24 h phenol photooxidation processes. A photocatalyst (50 mg) was suspended in an aqueous solution (250 cm3) containing 10 ppm phenol solution, the magnetic stirring was fixed at 500 rpm. Photodecomposition process was performed using halogen lamp with a power of 70 W (Philips Electronics North America Corporation, New York, NY, USA). The light photoirradiation (> 420 nm) was performed using a cutoff filter (Hoya Y44 Tokina Co. Ltd., Saitama, Japan) to eliminate UV light. The radiation intensity was of about 883 W/m2 vis. The concentration of phenol in the solution as well as the concentration of total organic carbon (TOC) remained in the solution after the photodegradation process was measured. Prior to all the measurements of the phenol concentration, the solution was filtered through a membrane filter with 0.45 mm pore diameter.
Results and discussion
Characterization of the photocatalysts
E g values and photocatalytic activity under UV-vis and visible light irradiation
UV-vis light irradiation
Visible light irradiation
Rate constant - k
TOC removal (%)
Photocatalytic activity of the materials
The photocatalytic activity of new materials was tested as described in experimental section and results are presented in Table 1. The phenol degradation kinetics under UV-vis and visible light were pseudo first-order, the rate constants (k) were obtained by fitting the experimental data. Functionalization of TiO2 with graphitic carbon resulted in the increase of photoactivity under visible light, whereas the UV-vis light activity decreased in comparison to starting material. Under visible light irradiation, TOC removal rate increased from 4% for pristine material to ca. 9% for samples heated at 400°C or 500°C with very short (10 s) deposition time. This relation is almost identical for both types of irradiations. Thus, the photoactivity decreases in samples being treated longer in CVD. Additionally, the effect of photoactivity is also more pronounced in case of materials prepared at 500°C in comparison to the materials treated at 400°C. These results are in agreement with their values of E g listed in Table 1. As it was mentioned above, the lower band-gap energy is not a key factor for the visible light applications. The photocatalytic activity depends on the efficiency of utilization of the fraction of the incident radiation absorbed by the catalyst, which results in the generation of electron/hole couples. These couples, though, may undergo a rapid recombination, resulting in neglecting quantum efficiency. Nevertheless, in case of presented materials, it is observed that higher visible light absorption and lower E g values indicate higher visible light photoactivity. Thus, these results prove, which was earlier predicted by other researchers , that recombination of electron/hole can be retarded by graphene. Lee et al.  described in details the reasons of graphene recombination inhibition. Graphene may work as electron acceptor and transporter. Due to its two-dimensional π-conjugation structure, it may also work as an acceptor of photogenerated electrons. On the other hand, due to its high conductivity, effective charge separation may be accomplished through electrons transport. It is also proved here that the carbon layer can also consist of thin layer of graphite and the above described effects are still observed.
In summary, we have successfully modified TiO2 nanocrystals with graphitic carbon via CVD. Here, acetylene was used as a carbon feedstock and TiO2 as a substrate. The investigations indicate that higher CVD temperature and longer time of reaction resulted in enhanced deposition of carbon. The amount of deposited carbon layer blue shifted the band-gap energy of the samples in comparison to the pristine TiO2. Photocatalytic activity of the materials was explored and it was found that TiO2 coated by thin graphitic layer exhibits higher photoactivity under visible light and lower activity under UV-vis light irradiation. Interestingly, samples being treated in CVD for a shorter time and higher temperature showed significantly better activity in the visible region.
- Pekakis PA, Xekoukoulotakis NP, Mantzavinos D: Treatment of textile dyehouse wastewater by TiO2photocatalysis. Water Res 2006, 40: 1276. 10.1016/j.watres.2006.01.019View ArticleGoogle Scholar
- Taranto J, Frochot D, Pichat P: Photocatalytic air purification: comparative efficacy and pressure drop of a TiO2-coated thin mesh and a honeycomb monolith at high air velocities using a 0.4 m3 close-loop reactor. Sep Purif Technol 2009, 67: 187. 10.1016/j.seppur.2009.03.017View ArticleGoogle Scholar
- Khan U, Benabderrazik A, Bourdelais AJ, Baden DG, Rein K, Gardinali PR, Arroyo L, O'Shea KE: UV and solar TiO2photocatalysis of brevetoxins (PbTxs). Toxicon 2009, 55: 1008.View ArticleGoogle Scholar
- Robertson PKJ, Robertson JMC, Bahnemann DW: Removal of microorganisms and their chemical metabolites from water using semiconductor photocatalysis. J Hazard Mater 2011. doi:10.1016/j.jhazmat.2011.11.058 doi:10.1016/j.jhazmat.2011.11.058Google Scholar
- Hirai T, Suzuki K, Komasawa I: Preparation and photocatalytic properties of composite CdS nanoparticles-titanium dioxide particles. J Colloid Interface Sci 2001, 244: 262. 10.1006/jcis.2001.7982View ArticleGoogle Scholar
- Kamat PV: Photoinduced transformations in semiconductor metal nanocomposite assemblies. Pure Appl Chem 2002, 74: 1693. 10.1351/pac200274091693View ArticleGoogle Scholar
- Hwajin K, Wonyong C: Effects of surface fluorination of TiO2on photocatalytic oxidation of gaseous acetaldehyde. Appl Catal B Environ 2007, 69: 127. 10.1016/j.apcatb.2006.06.011View ArticleGoogle Scholar
- Chen JH, Jang C, Xiao S, Ishigami M, Fuhrer MS: Intrinsic and extrinsic performance limits of graphene devices on SiO2. Nat Nanotechnol 2008, 3: 206. 10.1038/nnano.2008.58View ArticleGoogle Scholar
- Zhou Z, Wu J, Wang Z: Field emission from in situ-grown vertically aligned SnO2 nanowire arrays. Nanoscale Res Lett 2012, 7: 117. 10.1186/1556-276X-7-117View ArticleGoogle Scholar
- Mnatsakanov TT, Levinshtein ME, Pomortseva LI, Yurkov SN, Simin GS, Khan MA: Carrier mobility model for GaN. Solid State Electron 2003, 47: 111. 10.1016/S0038-1101(02)00256-3View ArticleGoogle Scholar
- Lee C, Wei X, Kysar JW, Hone J: Measurements of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321: 385. 10.1126/science.1157996View ArticleGoogle Scholar
- Balandin AA, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, Lau CN: Superior thermal conductivity of single-layer graphene. Nano Lett 2008, 8: 902. 10.1021/nl0731872View ArticleGoogle Scholar
- Zhang H, Lv X, Li Y, Wang Y, Li J: P25-graphene composite as a high performance photocatalyst. Acs Nano 2010, 4: 380. 10.1021/nn901221kView ArticleGoogle Scholar
- Wang Y, Shi R, Lin J, Zhu Y: Significant photocatalytic enhancement in methylene blue degradation of TiO2photocatalysts via graphene-like carbon in situ hybridization. Appl Catal B Environ 2010, 100: 179. 10.1016/j.apcatb.2010.07.028View ArticleGoogle Scholar
- Liu Z, He D, Wang Y, Wu H, Wang J: Graphene doping of P3HT:PCBM photovoltaic devices. Synth Met 2010, 160: 1036. 10.1016/j.synthmet.2010.02.022View ArticleGoogle Scholar
- Guskos N, Guskos A, Typek J, Berczynski P, Dolat D, Grzmil B, Morawski A: Influence of annealing and rinsing on magnetic and photocatalytic properties of TiO2. Mater Sci Eng B doi:10.1016/j.mseb.2011.10.017 doi:10.1016/j.mseb.2011.10.017Google Scholar
- Ferrari AC, Robertson J: Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000, 61: 14095. 10.1103/PhysRevB.61.14095View ArticleGoogle Scholar
- Worsley KA, Ramesh P, Mandal SK, Niyogi S, Itkis ME, Haddon RC: Soluble graphene derived from graphite fluoride. Chem Phys Lett 2007, 445: 51. 10.1016/j.cplett.2007.07.059View ArticleGoogle Scholar
- Dolat D, Quici N, Kusiak-Nejman E, Morawski AW, Puma GL: One-step, hydrothermal synthesis of nitrogen, carbon Co-doped titanium dioxide (N,C-TiO2) photocatalysts. Effect of alcohol degree and chain length as carbon dopant precursors on photocatalytic activity and catalyst deactivation. Appl Catal B Environ doi:10.1016/j.apcatb.2011.12.007 doi:10.1016/j.apcatb.2011.12.007Google Scholar
- Wang F, Zhang K: Reduced graphene oxide-TiO2nanocomposite with high photocatalystic activity for the degradation of rhodamine B. J Mol Catal A Chem doi:10.1016/j.molcata.2011.05.026 doi:10.1016/j.molcata.2011.05.026Google Scholar
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